The invention relates to an X-ray detector, in particular for X-ray photography and X-ray fluoro-chemical analysis, which X-ray detector comprises a scintillator containing a doped alkali halogenide and an array of photodiodes including at least one photodiode containing a semiconductor material.
In X-ray photography and X-ray fluoro-chemical analysis, X-radiation is analyzed which is emitted by an X-ray source and, on passing through the human body or a material, absorbed and attenuated in dependence upon the density of said human body or said material. X-radiation can be subjected to further processing in various ways. In an X-ray detector, the X-ray shadow image obtained is first absorbed by a scintillator and transposed in the scintillator into less energy-rich luminescent radiation in the visible range or the UV-range. This luminescent radiation is subsequently transmitted to an array of photodiodes. In the photodiodes, the luminescent radiation is converted to an electric signal, which can be subjected to further processing by means of readout electronics. The combination of a scintillator and an array of photodiodes thus enables a digital image to be obtained of the incident X-radiation.
Doped alkali halogenides can very suitably be used as the scintillators for X-ray detectors because they exhibit a high mass extinction coefficient for X-radiation. X-ray detectors comprising such scintillators are disclosed in DE 195 19 775.
The spectral distribution and the light output of the luminescent light generated in the scintillator depends on the doping selected for the alkali halogenides.
Also the array of photodiodes succeeding the scintillator do not act uniformly over the appropriate frequency range of the luminescent light. Certain frequency ranges of the luminescent light exhibit a high response threshold in the photodiodes, leading to a reduction of the sensitivity of the X-ray detector in this range. As a result, the wavelength of the luminescent light of the scintillator generally is outside the maximum of the photosensitivity of the photodiodes.
It is an object of the invention to provide an X-ray detector comprising a scintillator including a doped alkali halogenide, and comprising an array of photodiodes including at least one photodiode containing a semiconductor material, which X-ray detector exhibits a high efficiency.
In accordance with the invention, this object is achieved by an X-ray detector comprising a scintillator including a doped alkali halogenide, and comprising an array of photodiodes including at least one photodiode containing a semiconductor material, wherein a color transformer containing a photoluminescent phosphor is arranged between the scintillator and the array of photodiodes.
The photoluminescent phosphor contained in the color transformer enables a larger proportion of the X-radiation to be used for image analysis. The phosphor absorbs the luminescent radiation and converts it to luminescent radiation that is adapted to the spectral sensitivity of the photodiode, so that the emission spectrum of the scintillator and the sensitivity spectrum of the photodiode demonstrate a maximum overlap, and the photodiode operates with maximum quantum efficiency.
In accordance with a modification of the invention, the color transformer may contain two or more photoluminescent phosphors. This is useful in the event that the color-transformation effect of a single phosphor is insufficient. In this case, the combination of a plurality of phosphors enables a cascade effect to be attained, causing the luminescent radiation to be transferred to the desired wavelength range.
In accordance with a preferred embodiment of the invention, the scintillator comprises a doped alkali halogenide having an emission maximum at a wavelength xcex in the range from 400 to 440 nm.
The scintillator very preferably contains CsI:Na as the doped alkali halogenide. CsI:Na emits a high-energy luminescent spectrum in the wavelength range between 400 and 440 nm, which luminescent spectrum has a short relaxation time; the images produced are free of memory effects and losses due to radiation-free conversion processes.
In a further embodiment in accordance with the invention, the photodiode may contain amorphous silicon as the semiconductor material. In the case of photodiodes of amorphous silicon, particularly in combination with a scintillator of CsI:Na or CsI:CO32xe2x88x92, it proved to be very advantageous to use a green phosphor for the photoluminescent phosphor.
For the green phosphor use can very suitably be made of a perylene derivative, SrGa2S4:Eu, ZnS:Cu,Au, BaMgAl10O17:Eu,Mn or YAG:Ce.
In a very preferred embodiment, the array of photodiodes is a large-surface matrix-addressed image sensor on a TFT matrix.